It's a holdover from cursive.

The general rhythm in cursive hands is up-down-up-down, where downstrokes are letters and upstrokes connect the downstrokes. The downstrokes may be straight (as in t, l) or elliptical (as in a, o).

Some letters combine upstrokes and downstrokes, but these still alternate. For example, a d occurring at the beginning or in the middle of a word will be started just like a. The upstroke immediately following it will be made vertically, moving the pen into position for the long stem. Then one last downstroke will finish the stem. At the bottom of this stroke, you curl into the next upwards stroke, which leads into the following letter.

Some joins would be made horizontally, not vertically—as, for instance, an o leading into an a—but the rhythm is the same.

There are a few reasons why this pattern holds. One is that, using 'nibbed' quills, reed pens, and steel pens, pressure could be placed on downstrokes but not upstrokes. If you put too much pressure on the nib on your upstroke, it would dig into the paper and jam. So as the Roman alphabet continued to evolve in countries which used these tools, it was only natural that daily writing styles with bold downstrokes would be popular, as these could be put into use quickly and without fuss.

As for this:

Why was it so important to be taught to write letters from the top back in school?

I don't remember it being 'so important' in 1990. I think we probably were taught to write most letters from the top, but I don't remember an emphasis being placed on it. Depending on where you live, what curriculum your school board or national government prescribes, etc., you might not be learning the same system as everyone else.